Oil sand generally comprises water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules which contain a significant amount of sulfur, nitrogen and oxygen. The extraction of bitumen from sand using hot water processes yields large volumes of fine tailings composed of fine silts, clays, residual bitumen and water. Mineral fractions with a particle diameter less than 44 microns are referred to as “fines.” These fines are typically clay mineral suspensions, predominantly kaolinite and illite.
The fine tailings suspension is typically 85% water and 15% fine particles by mass. Dewatering of fine tailings occurs very slowly. When first discharged in ponds, the very low density material is referred to as thin fine tailings. After a few years when the fine tailings have reached a solids content of about 30-35%, they are referred to as mature fine tailings (MFT) which behave as a fluid-like colloidal material. Such fine tailings are generally referred to herein as fluid fine tailings. The fact that fluid fine tailings (FFT) behave as a fluid and have very slow consolidation rates significantly limits options to reclaim tailings ponds.
One approach to disposal/management of FFT is the Composite Tails (CT) process, which involves mixing a coarse tailings stream (e.g., sand) with an FFT stream and adding a coagulant such as gypsum to form slurry that rapidly releases water when deposited and binds the FFT in a coarse tailings/FFT deposit. Thus, more of the fines can be stored in a geotechnical soil matrix, which reduces the inventory of fluid-fine tails and enables a wider range of reclamation alternatives. Thus, CT causes the tailings to settle faster, enabling the development of landscapes that support grass, trees and wetlands. Composite tailings are often referred to as “non-segregating” tailings, meaning that the fines do not readily separate from the coarser sand.
There are currently two primary methods for discharging CT into bodies of water. The first is to discharge the CT using a floating pipeline. The second is to overboard off a pit wall and allow the CT to cascade into the water. However, neither of these techniques addresses the need to reduce the slurry velocities below the dynamic segregation limits. Nor do these techniques minimize the water dilution effects of exposing the CT slurry to water. As a result, the majority of CT placed using the aforementioned two methods have a high propensity to segregate.
Accordingly, there is a need for an improved method of discharging oil sand tailings into a body of water to reduce segregation of the fine solids and the coarse solids present in the oil sand tailings.